No, not unless you've made some provision to back up the old version.
For example, if you had checked the option "Always create backup copy"
in Tools > Options > Save, there would be a copy of the previous
version in a file named "Backup of YourDoc.wbk" in the same folder.
If that option wasn't checked, you could still get back the previous
version *only if you haven't already closed the document* by clicking
Undo to remove your changes. Once you close the document, though, the
Undo memory is gone.
See
http://www.gmayor.com/automatically_backup.htm and
http://www.word.mvps.org/FAQs/General/AutomaticSave.htm for
explanations of some other things you can do to prevent the same
problem from happening again.